1. Field
This invention relates to electromagnetic shielding and, in particular, to absorptive electromagnetic shielding intended for applications in high speed computers and other circuitry where fast rise times are present.
2. Prior Art
Electromagnetic interference (EMI) is unintentional radiation from electronic equipment which can interfere with external equipment such as radio, television or computer units, as well as interfere with internal circuitry within the unit generating the EMI. EMI has become increasingly important because of the substantial growth in the volume of electronic equipment, much of which is capable of either generation or being interferred with by EMI. The importance of EMI in the computer field has increased in part because of the increase in the speed of operation of modern computers which manifest itself electrically within the computer as signals with rise times as short as two nanoseconds. Such fast rise times are a natural source of wide spectrum interference. One of the most serious EMI problems is the interference caused by computers to their own operation. Computers may affect the operation of other computers positioned nearby by direct radiation and may also interfere with other computers which are remotely located by transmitting EMI along the interconnection lines intended to transmit data.
Prior art techniques for containing electromagnetic interference have been confined essentially to placing shielding boxes about the equipment producing the interference. Typical examples of prior art electromagnetic interference shielding are shown in FIGS. 1A, 1B and 1C. FIG. 1A illustrates a double screen shield in which the first screen is designated 101 while the second is designated 102. This type of shield is primarily used about screen rooms. FIG. 1B shows a solid sheet of conducting materials such as aluminum, designated by drawing numeral 103. This type of shielding is used for screen rooms and in shielding boxes about electronic equipment. FIG. 1C shows a tubular shield 104 made of conducting material such as aluminum, typically used about cable connections.
A primary problem with prior art shielding is the screen, sheet and tubular shield are incapable of absorbing EMI. These shields merely reflect the EMI, causing interference to the circuits which have generated the EMI. In many cases, the shielding boxes used in prior art systems must include provisions for cable connections to enable the equipment contained within these boxes to function. Although the EMI may be prevented from radiating directly through the shielding boxes, it can be carried out of the boxes on the cables to interfere with external equipment.
A third problem encountered with conventional shielding is illustrated by the graph shown in FIG. 1D. In this graph, the ordinate 106 indicates shielding attenuation in decibels while the abscissa 105 indicates frequency ranging from hertz through megahertz. Three solid line graphs 107 represent the H-field of copper, aluminum and steel while three dashed line graphs 108 represent the E-field of copper, aluminum and steel. It can be seen that the H-field attenuation at low frequency in the hertz range is almost negligible, while at high frequencies in the range of hundreds of megacycles the E-field attenuation is also negligible. It is apparent from these plots that low frequency H-field radiation can pass through conventional shielding as can high frequency E-field radiation. The high frequency E-field signals are easily generated in computers which are now operating with clock rates of 100 MHz and rise times in the order of two nanoseconds. In addition, many computers also produce the low frequency H-fields which penetrate conventional shielding as well.
A fourth problem is in the fabrication of prior art shielding boxes with provisions enabling them to be easily removed for servicing while at the same time avoiding a seam or opening through which EMI may be radiated.